On Tue, Jan 27, 2015 at 02:35:50PM -0800, Guido van Rossum wrote:
I'm still confused. We're talking about a function that compares two simple values, right? Where does the sequence of values come from? Numpy users do everything by the array, but they already have an isclose(). Or perhaps you're talking about assertApproxEqual in test_statistics.py? That has so many ways to specify the relative and absolute tolerance that I give up understanding it.
Those ways evolved from my actual use of the function. What I found in practice is that within each TestCase, most of the tests used the same values for error tolerences. At the very least, I was aiming for all the tests to use the same error tolerance, but in practice I never quite achieved that. From time to time I would have a particularly difficult calculation that just wouldn't meet the desired tolerance, and I had to accept a lower tolerance. Most individual test methods within a single TestCase used the same error tolerances, which I set in the setUp method as self.rel for relative error (or self.tol for absolute), and wrote: self.assertApproxEqual(x, y, rel=self.rel) over and over again, occasionally overriding that value: self.assertApproxEqual(x, y, rel=1e-6) Since most of the time, the method was taken the tolerances from self, I reasoned that I should just make the default "take the tolerances from self" and be done with it. So that's how the method evolved. Most of those tests were for functions that didn't end up in the std lib, so I'm not surprised that this was not so clear. If I remember correctly, at the time I was also reading a lot about OOP design principles (as well as stats text books) and I think the use of instance attributes for the default error tolerances was probably influenced by that. -- Steve